A dumbo drop is similar to a rusty trombone. It is where you are receiving a handjob while the person is licking or eating out your asshole. However, what makes it a dumbo drop is the act of defecating while they are doing this.

Man, this girl last night was nasty. While she was giving me a rusty trombone I had to shit so I decided... fuck it... I'll just dumbo drop her.